This map is disturbing, once you understand it. It's a new attempt to visualize an old problem — the shrinking of underground water reserves, in most cases because farmers are pumping out water to irrigate their crops.

The map itself isn't hard to grasp. The colored areas show the world's largest aquifers — areas which hold deposits of groundwater. The blue ones are doing fine; more rainfall is flowing into them than is being pumped out of them for homes or irrigating fields. As a result, these aquifers can continue to play a vital role in the environment. (Water in most aquifers doesn't just sit there. It flows slowly, underground, and ends up sustaining rivers and lakes and all the creatures who live there.)

The aquifers that are painted red, orange, or yellow, meanwhile, are being drained rapidly. How rapidly? That brings us to the complicated part of this graphic.

See those large grey shapes, below the map? Each one is a magnified reflection of an over-exploited aquifer. The amount of magnification represents the amount of water that people are currently pumping out of that aquifer, compared to the rate of natural replenishment. Tom Gleeson, at Montreal's McGill University, and Ludovicus P. H. van Beek, at Utrecht University, in the Netherlands, created this graphic for an article they published in this week's issue of the journal Nature.

They call those magnified shapes the "groundwater footprint" of each aquifer's exploitation. The footprint of the Upper Ganges aquifer, for instance, is 54 times bigger than the aquifer itself. Think about that footprint this way: It's the size, on a map, of the area that would be required to catch enough rainfall to replenish that aquifer and make up for all the water currently being pumped out of it.

Some of these aquifers are being exploited at a stunning rate, but what's truly alarming is how many people depend on that over-exploitation for their food. These aquifers include the Upper Ganges, covering densely populated areas of northern India and Pakistan, and the North China plain, which is the heart of corn-growing in that country. The aquifer of Western Mexico has become a large source of fruit and vegetable production for the U.S.

The High Plains aquifer in the United States, meanwhile, is having a particularly bad year. Farmers are pumping even more than usual, because of the drought afflicting this part of the country, and it is getting less replenishment from rainfall. So water levels in the aquifer are falling even faster, leaving less water for the region's rivers, birds, and fish.

This can't go on forever. Already, many farmers are being forced to dig deeper wells to get at that water. But bigger changes are on the way: New irrigation technologies that use water much efficiently; a shift to different crops that demand less water; and in some areas, they'll just have to stop using those underground stores of water altogether.

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